Re: openssh: Enabling sftp, but disabling ssh?

On Tue, Sep 05, 2006 at 10:57:45PM +0200, Tevfik Karagülle wrote:
> Alternative 1:
>
> > > Create a file named sftponly in bin directory:
>
> #!/bin/bash 
> if [ "$*" != "-c /bin/sftp-server" ]; then 
> echo "SFTP only!" 
> exit 1 
> fi 
> exec $@ 

Please understand, THIS WILL NOT WORK.  Under some circumstances it
may be possible for the user to send the shell signals which will
break out of the script and give the user a shell.  I saw a
demonstration of this which is one of the things that prompted me to
write rssh.

Also, you've invoked the shell as bash, which will cause the user's
environment files to be sourced, even if the system does not treat the
shell script as a login shell (which it may, I don't recall off hand).
The user could upload a .bashrc and/or .profile which contains
arbitrary commands, including something like this:

  exec /usr/bin/my_favorite_shell

A shell script can not ever reliably be used to protect an account
this way, except against naive users.

-- 
Derek D. Martin
http://www.pizzashack.org/
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